1. Field of the Invention
This invention relates to fired heaters, also known as process furnaces, and more specifically to fired heaters used in processing hydrocarbons.
2. Description of the Related Art
Typical fired heaters are designed to heat hydrocarbons. Numerous processes on hydrocarbons are carried out in furnaces commonly known as fired heaters, or process furnaces, or fired heater furnaces, pipe stills.
Fired heaters are equipment in which fluid is heated to high temperatures by burning fuel gas or fuel oil in a combustion chamber. The tubes carrying the fluid are located in the center or on sides in the combustion chamber. The combustion chamber is lined with refractory material. The hot flue gases in the vicinity of the burners transmit heat to the fluid feed primarily by radiant heat transfer mechanism. This part of the heater is known as the radiant section or firebox section. The flue gases leaving the radiant section are typically at 1400–1800° F. and more heat can be recovered from these gases. Additional heat is recovered in the convection section where the flue gases are cooled by exchanging the heat with the fluid. In heaters, fluid generally enters the convection section first and then flows through the radiant section to maximize the heat recovery. In some heaters, process fluid enters through the radiant section and leaves through the radiant section. In these heaters, heat in the convection section is recovered by generating steam or preheating other hydrocarbon services. Flue gases are disposed off to the atmosphere through a stack.
Most refineries possess catalytic reforming units. In these catalytic reforming units, a hydrocarbon, for example, light petroleum distillate (naphtha) is contacted with platinum catalyst at elevated temperature and pressure. This process produces high-octane liquid product that is rich in aromatic compounds. The process upgrades low octane number straight run naphtha to high-octane motor fuels. In a typical unit, the feed to the unit is mixed with recycle hydrogen gas and it is heated first in heat exchangers and then in a fired heater. The feed is then sent to a reactor. Most reactions that occur in the reactor are endothermic reactions and occur in stages. The reactors are separated into several stages. Inter stage heaters may be installed between the reaction stages to maintain the desired temperature of the hydrocarbon feed.
Refineries have been de-bottlenecking their units to improve the fired heater capacity and improve thermal efficiency of the system. FIG. 1 illustrates the commonly practiced concept of the technique (prior art) used for heating the feed. A typical existing unit 100 comprises a convection section 120 and a radiant section 150. The feed is first sent to the convection section 120 through a plurality of fluid passes 122, 124, 126, 128, and 130, comprising fluid oath 135 for example. The preheated fluid then enters the radiant section 150 where it is heated further and the fluid exits through a fluid exit path 140. The fluid exiting the fluid path 140 may then be further passed through a series of concatenated fired heaters similar to the fired heater system 100.
Alternatively, the fluid may be introduced directly into the radiant section or in the convection section. Typically, when the fluid is directly introduced in the radiant section, a significant a mount of heat energy remains in the flue gases. A portion of this remaining energy may be recovered in the convection section by generating steam, preheating combustion air, or preheating other streams. Often times the refiners do not need the steam and they do not have other attractive choices.
In such fired u nits, the feed consists of hydrocarbon vapors and recycle hydrogen gas. The feed in vapor form has a very large volume and pressure drop across the heater is very important. Low-pressure drop minimizes recycle gas compressor differential pressure and the necessary compressor horsepower. The result is lower utility consumption. Low-pressure drop also permits operation at lowest reactor pressure. As a result, the heaters are designed as all radiant heaters with large manifolds at the inlets and outlets. Convection sections are typically used for steam generation or other waste heat recovery operations. Often times, the byproducts of waste heat recovery are not needed, and the heat is discharged in to the atmosphere.